A Horsa report that I have has a drawing of a Bren gun mounted in the roof escape hatch aft of the wing as well as the belly position, mind you it also has the troop seats replaced by fuel tanks!
The belly gun position was used, but not for its original intended use by the Horsas that went to the Orne bridges. The hatch was where the braking parachutes were deployed from.
To see this hatch in the flesh you should try and get along to see the Horsa replica being built at RAF Shawbury by the Assault Glider Trust.